michael@0: // Copyright (c) 2011 The Chromium Authors. All rights reserved.
michael@0: // Use of this source code is governed by a BSD-style license that can be
michael@0: // found in the LICENSE file.
michael@0: 
michael@0: #include "base/lazy_instance.h"
michael@0: 
michael@0: #include "base/at_exit.h"
michael@0: #include "base/atomicops.h"
michael@0: #include "base/basictypes.h"
michael@0: #include "base/threading/platform_thread.h"
michael@0: #include "base/third_party/dynamic_annotations/dynamic_annotations.h"
michael@0: 
michael@0: namespace base {
michael@0: namespace internal {
michael@0: 
michael@0: // TODO(joth): This function could be shared with Singleton, in place of its
michael@0: // WaitForInstance() call.
michael@0: bool NeedsLazyInstance(subtle::AtomicWord* state) {
michael@0:   // Try to create the instance, if we're the first, will go from 0 to
michael@0:   // kLazyInstanceStateCreating, otherwise we've already been beaten here.
michael@0:   // The memory access has no memory ordering as state 0 and
michael@0:   // kLazyInstanceStateCreating have no associated data (memory barriers are
michael@0:   // all about ordering of memory accesses to *associated* data).
michael@0:   if (subtle::NoBarrier_CompareAndSwap(state, 0,
michael@0:                                        kLazyInstanceStateCreating) == 0)
michael@0:     // Caller must create instance
michael@0:     return true;
michael@0: 
michael@0:   // It's either in the process of being created, or already created. Spin.
michael@0:   // The load has acquire memory ordering as a thread which sees
michael@0:   // state_ == STATE_CREATED needs to acquire visibility over
michael@0:   // the associated data (buf_). Pairing Release_Store is in
michael@0:   // CompleteLazyInstance().
michael@0:   while (subtle::Acquire_Load(state) == kLazyInstanceStateCreating) {
michael@0:     PlatformThread::YieldCurrentThread();
michael@0:   }
michael@0:   // Someone else created the instance.
michael@0:   return false;
michael@0: }
michael@0: 
michael@0: void CompleteLazyInstance(subtle::AtomicWord* state,
michael@0:                           subtle::AtomicWord new_instance,
michael@0:                           void* lazy_instance,
michael@0:                           void (*dtor)(void*)) {
michael@0:   // See the comment to the corresponding HAPPENS_AFTER in Pointer().
michael@0:   ANNOTATE_HAPPENS_BEFORE(state);
michael@0: 
michael@0:   // Instance is created, go from CREATING to CREATED.
michael@0:   // Releases visibility over private_buf_ to readers. Pairing Acquire_Load's
michael@0:   // are in NeedsInstance() and Pointer().
michael@0:   subtle::Release_Store(state, new_instance);
michael@0: 
michael@0:   // Make sure that the lazily instantiated object will get destroyed at exit.
michael@0:   if (dtor)
michael@0:     AtExitManager::RegisterCallback(dtor, lazy_instance);
michael@0: }
michael@0: 
michael@0: }  // namespace internal
michael@0: }  // namespace base